A. Field of the Invention
Embodiments of the present invention relate generally to systems and methods for extracting lipids of varying polarity from a wet oleaginous material, including for example, an algal biomass. In particular, embodiments of the present invention concern the ability to both extract & fractionate the algae components by doing sequential extractions with a hydrophilic solvent/water mixture that becomes progressively less polar (i.e. water in solvent/water ratio is progressively reduced as one proceed from one extraction step to the next). In other words, the interstitial solvent in the algae (75% of its weight) is water initially and is replaced by the polar solvent gradually to the azeotrope of the organic solvent. This results in the extraction of components soluble in the polarity developed at each step, thereby leading to simultaneous fractionation of the extracted components.
B. Description of Related Art
Algae have gained significant importance in the recent years given their inherent advantage in solving several critical issues of the world such as producing renewable fuels, reducing global climate change, wastewater treatment and sustainability. Algae's superiority as a biofuel feedstock arises from a variety of factors, viz, high per-acre productivity compared to typical terrestrial oil crop plants, non-food based feedstock resources, use of otherwise non-productive, non-arable land, utilization of a wide variety of water sources (fresh, brackish, saline, and wastewater), production of both biofuels and valuable co-products. However, the ability to easily recover and fractionate the various oil/byproducts produced by algae is critical to the economic success of the algae oil process.
Several thousand species of algae have been screened and studied for lipid production worldwide over the past several decades of which about 300 rich in lipid production have been identified. The lipids produced by algae are similar in composition compared to the contemporary oil sources such as oil seeds, cereals, and nuts. The lipid composition and content vary at different stages of the life cycle and are affected by environmental and culture conditions. Given considerable variability in biochemical composition and the physical properties of the algae cell wall, the strategies and approaches for extraction are rather different depending on individual algal species/strains employed. The conventional physical extraction processes such as extrusion, do not work well with algae given the thickness of the cell wall and the small size (2˜20 nm) of algal cells. Further, the large amounts of polar lipids in the algal oil compared to the typical oil seeds lead to refining issues. However, this can be a great opportunity to recover large amounts of polar lipids which have an existing market and add value to the process.
Typical algal concentration in the culture upon harvesting is about 0.1˜1.0% (w/v), thereby requiring the process to remove as high as 1000 times the amount of water to process a unit weight of algae. Conventional or the currently existing oil extraction methods for oleagenous materials strictly require almost completely dry biomass or feed to improve the yield and quality of the oil extracted, thereby rendering the feed to the biofuels process uneconomical and energy-intensive. The feed is extruded or flaked at high temperatures to enhance the extraction. These steps may not work with the existing equipment due to the single cell micrometric nature of algae. Algal oil extraction can be classified as disruptive and non-disruptive methods. Disruptive methods involve cell lysis by mechanical (see U.S. Pat. No. 6,750,048), thermal, enzymatic or chemical methods. Most disruptive methods result in emulsions and require an expensive cleanup process. Algal oils contain a large percentage of polar lipids and proteins which enhance the emulsification of the neutral lipids further stabilized by the nutrient and salt components left in the solution. The resulting oil is a complex mixture requiring an extensive refining process to obtain neutral lipids (feed for conversion to biofuels).
Non-Disruptive methods provide low yields. Milking is a variant of the proposed process. However, it may not work with some species of algae due to solvent toxicity and cell wall disruption. A specific process may be required for each algal strain, mutant and genetic modified organism. Further, the volumes of solvents required would be astronomical due to the maximum attainable concentration in the medium. Multiphase extractions (see U.S. Pat. No. 6,166,231) will require extensive distillations with complex solvent mixtures for solvent recovery and recycle.
The proposed non-disruptive alcoholic extraction process results in over 90% extraction efficiency, and the small amount of polar lipids in the remaining biomass enhances its value. In addition, ethanol extracts can further be directly transesterified. Furthermore, it is a generic process for any algae, and recovers all the valuable components (polar lipids) in the algae with a gradient in alcohol-water mixture. The neutral lipids fraction has a low metal content to start with, thereby enhancing the stability and improving process economics in the subsequent steps.
The proposed system and methods start with wet biomass, reducing the dying and dewatering costs. Compared to the contemporary processes, this process should have a relatively low operating cost due to the moderate temperature and pressure conditions along with the solvent recycle. In addition, continuous solvent extraction is a proven technology, and chlorophylls may be removed from the fuel-lipid fractions by solvent and solid interactions. Furthermore, the existing processes are cost prohibitive and cannot meet the demand of the market.
Another aspect of proposed systems and methods is the ability to separate the polar lipids from neutral lipids during the extraction process. The polar lipids along with metals result in processing difficulties for separation and utilization of neutral lipids. We take this opportunity to develop a value added aspect to the extraction process and at the same time separate the polar lipids. The polar lipids are surfactants by nature due to their molecular structure. The world market of surfactants reached $23.9 billion in 2008, growing steadily at about 2.8%. By the year of 2010, biosurfactants could capture 10% of the surfactant market, reaching $2 billion in sales (Nitschke et al., 2005). The annual surfactant market in the U.S. is about 7.7 billion pounds, of which 60% is oleoehemical based. These biosurfactants are either derived directly from the vegetable oil refining processes, or from oil seeds, bacteria and yeast by extensive separation processes or enzymatic esterification. There is a large existing surfactants market for phospholipids. The U.S. food industry consumes over 100 million pounds per year of lecithin (soybean phospholipid, an anionic surfactant). These are co-products of soybean and other vegetable oil refining processes. However, the amount of phospholipids in the initial crude oil is at the most 3% (i.e., 3000 ppm). Also, non-ionic synthetic surfactant consumption in the same market is four times the size of the lecithin market. Non-ionic biosurfactants such as glycolipids, if available in bulk, can potentially replace lecithin.
Some of the major glycolipid biosurfactants, rhamnolipids, sophorolipids, and trehalose lipids are produced by microbial fermentation. Rhamnolipids are produced intracellularly by the bacterium Pseudomonas sp. Sophorolipids are produced extracellularly by Candida sp. Trehalose lipids are cell wall components in Mycobacteria and Corynebacteria. These are major toxic components in the cell wall and reduce the permeability of the membranes conferring appreciable drug resistance to the organisms. These fermentation processes typically use hydrocarbons, glucose, vegetable oils as substrates (Gautam and Tyagi, 2006)
Recently the synthesis of biosurfactants has been developed using microbial enzymes. There have been many reports on the synthesis of sugar fatty acid esters from sugars (glucose, fructose and sucrose) and sugar alcohols (glycerol, xylitol and sorbitol) catalyzed by lipases (Kitamoto et al., 2002). In the lipase—catalyzed esterification, which is a dehydration condensation, one of the major difficulties is how to efficiently remove water produced as the reaction progresses or how to properly regenerate the solvent. Several strategies are being used to surmount these problems, namely to perform the reaction under reduced pressure, to use water adsorbents like molecular sieves, or to employ membrane pervaporation techniques (Yahya et al., 1998; Yan et al., 2001). Further, there is a problem with stability and activity of the enzyme, and the solubility of substrates (especially solubility of sugars in organic solvents). An example of the industrial production of glycolipid biosurfactants using the enzyme method is synthesis of a butyl glucoside from maltose and n-butanol by glucose transferase with an annual yield of 240 kg (Bonsuet et al., 1999).
All the existing technologies for producing polar lipids are raw material or cost prohibitive. Other economical alternative feedstocks for glycolipids and phospholipids are mainly algae oil, oat oil, wheat germ oil and vegetable oil. Algae oil typically has 30-85% (w/w) polar lipids depending on the species, physiological status of the cell, culture conditions, time of harvest, and the solvent utilized for extraction. The biosurfactant properties that enable numerous commercial applications also increase the separation costs and losses at every processing step. Because the glycerol backbone of each polar lipid has two fatty acid groups attached instead of three in the neutral lipid triacylglycerol, transesterification of the former may yield only two-thirds of the end product, i.e., esterified fatty acids, as compared to that of the latter, on a per mass basis. Hence, removal and recovery of the polar lipids would not only be highly beneficial in producing high quality biofuels or triglycerides from algae, but also generate value-added co-products glycolipids and phospholipids, which in turn can offset the cost associated with algae-based biofuel production.
Biosurfactant recovery depends mainly on its ionic charge, water solubility, and location (intracellular, extracellular or membrane bound). Examples of strategies that can be used to separate and purify polar lipids in batch or continuous mode include (Gautam et al., 2006): (1) Batch mode: Precipitation (pH, organic solvent), solvent extraction and crystallization; (2) Continuous mode: centrifuging, adsorption, foam separation and precipitation, membranes (tangential flow filtration, diafiltration and precipitation, ultra filtration)
Most of the above listed technologies were utilized in separation and purification of biosurfactants either from fermentation media or vegetable oils. However, exemplary embodiments of the present disclosure utilize a crude algal oil that is similar with a vegetable oil in terms of lipid and fatty acid composition. The differences between algal oil used in exemplary embodiments and vegetable oils used in previous embodiments include the percentage of individual classes of lipids. An exemplary algal crude oil composition is compared with vegetable oil shown in Table 1 below:
Algal Crude Oil (w/w)Vegetable Oil (w/w)Neutral lipids30-90%90-98%  Phospholipids10-40%1-2% Glycolipids10-40%<1%Free fatty acids 1-10%<3%Waxes 2-5%<2%Pigments 1-4%ppm
In the vegetable oil industry, the product of chemical degumming to remove polar lipids (biosurfactants) retains a lot of the neutral lipid (triglycerides) fraction. This neutral lipid fraction is further removed from the degummed material using solvent extraction or supercritical/subcritical fluid extraction or membrane technology. Of these technologies, membrane technology may eliminate the preliminary chemical degumming step and directly result in polar lipids almost devoid of neutral lipids.